1. Field of the Invention
The present invention relates to a washing system for washing semiconductor wafers and wafer cassettes.
2. Description of the Related Art
Semiconductor devices such as super LSIs have been more and more highly integrated and their circuit patterns have been micro-structured to an order of submicrons. In addition, the diameter of semiconductor wafers has been made large to 8 inches and even 12 inches. When such a small amount of particles that could be hitherto neglected adhere to semiconductor wafers, therefore, the productivity of wafers is remarkably lowered. In order to overcome this, the semiconductor wafers are immersed into acid and alkali solutions in process vessels so that their surfaces can be made clean.
The automatic washing system can be cited as a system for washing the surface of each wafer. It has plural process vessels in which the semiconductor wafers are washed by chemical solutions and water and then dried. Namely, an ammonium solution vessel, a water-washing vessel, a rinsing vessel, a hydrofluoric acid vessel and a drying chamber are arranged in it to enable a series of processes to be sequentially applied to the semiconductor wafers.
In the case of this automatic washing system, twenty five sheets of semiconductor wafers are housed in a cassette and they are immersed together with the cassette in the chemical solution in each process vessel. After they are immersed in this manner, the cassette is picked up out of the process vessels and washed by water and then dried.
However, the wafer cassette has a plurality of grooves in which wafers are seated, and the whole of the cassette is thus made complicated in shape. This makes it difficult to remove acid and alkali solutions from the cassette. When the cassette is repeatedly immersed in acid and alkali solutions, material (e.g. an ethylene fluoride system resin) of which the cassette is made solves little by little into the chemical solutions to degrade them in the process vessels.
According to the automatic washing system now used, therefore, the wafers are taken out of the cassette and are immersed and washed in the chemical solutions. The wafers which have been thus made clean are then returned into their original cassette.
In the case of the automatic washing system, however, there is fear that particles and dust which have stuck to the original cassette move to the wafers which have been made clean to contaminate them. The reason why particles and dust adhere to the cassettes is that they are contacted directly with and come near to various carrier units in the clean room and that particles and dust are caused at drive sections of these carrier units. The amount of particles and dust thus stuck to the cassettes reaches to such a value that cannot be neglected. In addition, oils and fats, chemical solutions and other matters sometimes adhere to the cassettes.
Further, when the wafers themselves come near to the carrier units, the particles and dust caused will adhere to them.